The present invention relates to an illumination optical system, and more particularly to control over an incident angle distribution or light distribution characteristic (also referred to as “effective light source” and “σ distribution”) on a surface to be illuminated (“target surface”). The inventive illumination optical system is suitable for an exposure apparatus for the micro-lithography used to manufacture fine patterns such as semiconductor devices, liquid crystal devices (“LCDs”) and magnetic materials.
A projection exposure apparatus has been conventionally employed which uses a projection optical system to project a circuit pattern of a reticle (or a mask) onto a wafer, etc. to transfer the circuit pattern, in manufacturing such fine semiconductor devices as a semiconductor memory and a logic circuit using the photolithography technology. Along with the recent demands for smaller and lower profile electronic apparatuses, the finer processing to the semiconductor devices mounted on the electronic apparatuses are increasingly required. One known means for achieving the high resolution is to increase the numerical aperture (“NA”) of the projection optical system (high NA scheme).
In addition, for the high-quality exposure, an effective light source should be optimized in accordance with a pattern of the surface to be illuminated, such as a reticle. For example, the effective light source distribution is implemented by adjusting an intensity distribution near a fly-eye lens's exit surface to a desired shape, such as a normal illumination condition, an annular illumination condition and a quadrupole illumination condition. A projection exposure apparatus is required to have a means for optimizing the NA of the projection optical system, a coherence factor σ that is the illumination optical system's NA/the projection optical system's NA and the effective light source to processes having various characteristics.
The optical path in the recent illumination optical system becomes longer as the required function diversifies, e.g., a function of forming various effective light sources. It is therefore difficult to arrange the illumination optical system along a straight line, and it is necessary to deflect the optical path using a deflection mirror so as to reduce the size of the exposure apparatus. One known means for monitoring the exposure dose is to provide the illumination optical system with the half-mirror and to monitor the transmitting light through or reflected light from the half-mirror.
When the incident exposure light spreads in the deflection mirror or the half-mirror, the incident angle of the light can differ according to locations in the mirror. The conventional design technology has maintained mirror's transmittance or reflectance fluctuations within the latitude, but recently had difficulties in doing so as the high NA scheme advances in accordance with the fine-processing request. In addition, a usable coating material is limited for use with the mirror and the light having a wavelength of 250 nm or smaller, and the design freedom is limited accordingly.
Therefore, a desired effective light source distribution could be obtained when no mirror is used, but when the mirror is used, its transmittance and reflectance characteristics preclude a formation of the desired effective light source distribution. This problem causes a pattern to be exposed with a coherence factor σ different from the optimized one that provides a transfer of the minimum critical dimension (“CD”) of the pattern, thereby preventing the designed resolution CD (in particular minimum CD) from being obtained. In addition, another problem of “HV difference” occurs which is a difference between horizontal and vertical CDs transferred on a wafer, lowering the yield.
Moreover, the contrast of the interference fringe of a line and space (“L & S”) pattern formed on the photosensitive agent lowers when the diffracted light from the L & S pattern is the p-polarized light. This decrease becomes striking as the high NA scheme proceeds. Accordingly, studied as a solution for this problem is a polarized illumination that utilizes the s-polarized light, in which a vibration direction of an electric field vector of the light is parallel to the wafer surface and perpendicular to the light traveling direction. Nevertheless, the s-polarized light and the p-polarized light have different transmittances and reflectances on the mirror and cause a similar HV difference.
Prior art for solving the non-uniformity of the transmittance distribution in the illumination optical system includes, for example, Japanese Patent Applications, Publication Nos. 2002-093700, 2003-243276, and 2002-75843.
While Japanese Patent Application, Publication No. 2002-093700 proposes a correction of the transmittance by adjusting an angle between two filters each having a discrete transmittance distribution, the angular adjustment requires a measurement of the actual effective light source and a long time. Another problem is that the discrete transmittance distribution cannot improve the correction accuracy. On the other hand, Japanese Patent Application, Publication No. 2003-243276 has a purpose to correct the non-uniformity of the transmittance caused by an incident position upon the lens, but does not consider the non-uniformities of the reflectance and transmittance caused by an incident angle upon a mirror.